1. Field of the Invention
The present invention relates to a control system for controlling a hybrid vehicle having an internal combustion engine and an electric motor as separate propulsion sources, and more particularly to a control system for controlling a hybrid vehicle to regenerate electric energy with an electric motor when the hybrid vehicle is decelerated.
2. Description of the Related Art
There have heretofore been known hybrid vehicles each having an internal combustion engine and an electric motor as separate propulsion sources. For accelerating such a hybrid vehicle, the drive axle of the hybrid vehicle is driven by the engine, and stored electric energy is supplied from an electric energy storage unit such as a battery to the electric motor to enable the electric motor to assist in rotating the drive axle. For decelerating the hybrid vehicle, the kinetic energy of the drive axle, i.e., the kinetic energy transmitted from the drive wheels of the hybrid vehicle to the drive axle, is supplied to the electric motor to regenerate electric energy, which is stored in the electric energy storage unit.
One known control system for such a hybrid vehicle is disclosed in Japanese laid-open patent publication No. 7-123509, for example.
According to the disclosed control system, the present capacity (remaining capacity) of a battery as an electric energy storage unit is detected from time to time, and an amount of electric energy that can be regenerated by an electric motor, i.e., an amount of regeneratable electric energy, until the hybrid vehicle comes to a stop is determined from the vehicle speed, etc. Upon acceleration of the hybrid vehicle when the throttle valve of the engine is opened beyond a predetermined opening value, if the sum of the remaining capacity of the battery and the amount of regeneratable electric energy is greater than a minimum capacity of the battery required for the electric motor to restart the engine, then the electric motor is energized to assist in rotating the drive axle.
When the brake pedal of the hybrid vehicle is depressed to brake the hybrid vehicle, if the sum of the remaining capacity of the battery and the amount of regeneratable electric energy is smaller than a reference capacity which is substantially equal to the capacity of the fully charged battery, then since the battery needs to be charged, the electric motor regenerates electric energy, which is stored in the battery. At this time, the amount of regenerated electric energy is controlled in proportion to the depth to which the brake pedal is depressed.
When the opening of the throttle valve is smaller than the predetermined opening value and the brake pedal is not depressed, if the sum of the remaining capacity of the battery and the amount of regeneratable electric energy is smaller than the reference capacity, then electric energy is generated by the electric motor and stored in the battery. The amount of electric energy that is generated by the electric motor is controlled depending on an amount of electric energy to be charged, which is represented by the difference between the sum of the remaining capacity of the battery and the amount of regeneratable electric energy and the reference capacity.
However, the above known control system has suffered the following drawbacks:
According to the above known control system, when the hybrid vehicle is decelerated by braking, the remaining capacity of the battery is taken into account for deciding whether the electric motor is to regenerate electric energy, but the amount of electric energy regenerated by the electric motor is proportional to the depth to which the brake pedal is depressed, irrespective of the remaining capacity of the battery. Therefore, if the depth to which the brake pedal is depressed is relatively small to brake the hybrid vehicle slowly, then the amount of regenerated electric energy is small. If the hybrid vehicle is temporarily decelerated, but not stopped, by braking, then the kinetic energy of the running hybrid vehicle cannot be converted into a sufficient amount of electric energy for storage in the battery. As a result, when the hybrid vehicle is to be accelerated after such temporary deceleration, no sufficient amount of electric energy can be supplied from the battery to the electric motor, and hence the electric motor fails to provide sufficient assistance in rotating the drive axle.
With the above known control system, while the accelerator pedal is being released in order to decelerate the hybrid vehicle, the electric motor regenerates an amount of electric energy depending on the amount of electric energy to be charged, which is referred to above, until the brake pedal is depressed. Therefore, when the hybrid vehicle decelerates in this manner, the amount of electric energy regenerated by the electric motor takes into account the remaining capacity of the battery.
However, as when the hybrid vehicle is decelerated by braking, the electric motor regenerates electric energy only when the sum of the remaining capacity of the battery and the amount of regeneratable electric energy, i.e., the amount of electric energy that can be regenerated by the electric motor until the hybrid vehicle is stopped from the present vehicle speed. Thus, when the hybrid vehicle runs at a high speed at which the amount of regeneratable electric energy is relatively large, no electric energy can be regenerated by the electric motor unless the hybrid vehicle decelerates to a sufficiently low vehicle speed. In this case, when the hybrid vehicle running at a high speed decelerates, the electric motor does not generate a regenerative braking torque, failing to maintain sufficient braking forces for the hybrid vehicle.